1. Field of the Invention
The present invention relates to microelectromechanical systems (MEMS) and, in particular, relates to the fabrication of MEMS structures.
2. Discussion of the Related Art
Microelectromechanical systems (MEMS) components are being progressively introduced into many electronic circuit as well as micro-sensor applications. Examples of MEMS components are electromechanical motors, radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. In one application, the MEMS device is an accelerometer having a movable component that, in response to an external stimulus, is actuated so as to vary the size of a capacitive air gap. Accordingly, the capacitance output of the MEMS device provides an indication of the strength of the acceleration.
When the MEMS device is an accelerometer, the device comprises a stationary MEMS element that is attached to a nonconductive substrate, and a movable MEMS element that has a substantial portion that is free from mechanical contact with the substrate that is therefore movable with respect to the stationary element.
One method of fabricating such components, often referred to as surface micro-machining, uses a sacrificial layer, such as silicon dioxide, that is deposited and bonded onto a substrate, such as single crystal silicon which has been covered with a layer of silicon nitride. A MEMS component material, for example polycrystalline silicon, is then deposited onto the sacrificial layer, followed by a suitable conductor, such as aluminum, to form an electrical contact with the ambient environment. The silicon layer is then patterned by standard photolithographic techniques and then etched by a suitable reactive ion etching plasma or by wet chemistry to define the MEMS structure and to expose the sacrificial layer, which may comprise silicon dioxide. The sacrificial layer is then etched to release the MEMS component.
Several disadvantages are associated with fabricating a MEMS device using a sacrificial layer. First, it requires the availability of an etching process that is capable of selectively etching the sacrificial layer without reacting with the other materials that will ultimately form the MEMS device. This limits the materials that may be used when fabricating the MEMS device. Additionally, the use of a sacrificial layer increases the amount of materials needed to form the MEMS device, thereby adding cost and complexity to the fabrication process. Furthermore, an additional etching step is needed to remove the sacrificial layer, thereby further reducing the efficiency of the fabrication process. In particular, because the structure forming the movable MEMS element is disposed on top of the sacrificial layer, a significant amount of time is needed to completely undercut the sacrificial layer. In fact, in some instances, holes are first etched through the base of the movable MEMS element in order to permit the etchant to access the sacrificial layer.
What is therefore needed is an improved reliable method for manufacturing isolated MEMS devices using simplified etching processes that avoids the disadvantages associated with undercutting a sacrificial layer to release the movable MEMS element.